Lithographic accuracy and throughput of a lithography machine are direct determinants for the integration and manufacturing costs of integrated circuit (IC) chips. However, the throughput and the lithographic accuracy are conflicting to each other. IC manufacturers are pursuing lithography machines with both a high-throughput capability and an ability to perform high-quality, high-accuracy exposures. The accuracy of the lithography machine will be affected and impaired by any vibrations, even tiny in magnitude, generated within the lithography machine, and will especially be affected by the reaction forces generated upon driving the object tables.
To overcome this problem, a technique that uses balance mass has been developed. This technique of balance mass can minimize the reaction forces from the object tables that are transferred to the base frame of the machine, thereby greatly reducing difficulties in vibration damping for the lithography machine and reliving the exposure system from the interference of the reaction forces.
The existing balance mass systems for object tables are mainly categorized into two types which are single-layered structure and double-layered structure. TwinScan lithographic apparatuses fabricated by ASML employ workpiece tables using the single-layered balance mass system, more information of which can be gleaned from U.S. Pat. No. 7,034,920, published on Apr. 25, 2006. This system utilizes a single-layered frame as a balance mass for balancing reactions from acceleration or deceleration movements of long-stroke motors in X, Y and Rz directions. In addition, this balance mass system for the workpiece table is in physical connection to the base frame via two five-rod mechanisms. These two five-rod mechanisms can accomplish position initialization and zeroing of the balance mass system of the workpiece table as well as tracking and correction of the balance mass system of the workpiece table during its movements. However, as the two five-rod mechanisms incorporate totally four control motors which act in concert to provide combined control in the X, Y and Rz degrees of freedom, this control scheme must involve a decoupling process and is hence associated with a high complexity.
Nikon's Tandem Stage provides an example of a two-stage table using a double-layered balance mass system, more information of which can be gleaned from U.S. Pat. No. 6,885,430, published on Apr. 26, 2005. The system has two layers of balance masses for respectively balancing reactions from acceleration or deceleration movements of long-stroke motors in X, Y and Rz directions, wherein an upper balance mass is configured to balance the reactions in one linear direction (X or Y) and reactions in the Rz direction. The upper and lower balance masses are connected by a linear motor which acts as a compensating motor to realize synchronized movements of the upper and lower balance masses with deviations compensated and corrected. The lower balance mass is configured to balance the reactions in the other linear direction (Y or X). The Tandem double-layered balance mass system employs three independent linear motors to connect the balance mass system of the workpiece table to the base frame. The stators of the linear motors are mounted on the base frame and the rotors are mounted on the lower balance mass of the workpiece table. These three linear motors can cooperate to accomplish the position initialization and zeroing of the balance mass system of the workpiece table as well as tracking and correction of the balance mass system of the workpiece table during its movements. As the three linear motors are capable of independent control in the X, Y and Rz degrees of freedom, this scheme does not need any decoupling process and thus allows easier control. However, the used linear motors are all of a special design of Nikon Corporation that takes in account backlash and displacement and could hence not be generalized to ordinary linear motors.
As described above, balance mass systems used in current lithography machines, represented by the ASML Twinscan system and Nikon Tandem system, all feature the use of independent balance mass systems for workpiece and mask tables, i.e., adoption of at least two such balance mass systems in the same lithography machine. This configuration is associated with the following drawbacks:
1) an overall structure lack of compactness;
2) necessity of using an additional support for the mask stage, which leads to an increase in both the size and mass of the lithography machine; and
3) a considerable total mass of used independent balance masses.
Therefore, there is an urgent need in this art for a balance mass system and a lithography machine with high structural compactness and a small size.